Recently, the use of multiple cold-cathode fluorescent lamps in a surface light source such as a liquid crystal display backlight becomes popular, which demands a high-power inverter circuit.
A high-power inverter circuit is generally realized by enlarging a step-up transformer and its drive circuit. Because even a slight power loss in a high-power inverter circuit leads to generation of large heat, a high efficiency inverter circuit is needed.
The present inventor has proposed in U.S. Pat. No. 5,495,405 (corresponding to Japanese Patent No. 2733817), as a high efficiency (a highly efficient) inverter circuit, a leakage flux transformer inverter circuit which utilizes an effect of improving the power factor as a result of reducing the exciting current flowing across the primary winding of a step-up transformer by resonating the secondary side circuit thereof.
Those high efficiency inverter circuits have been used as inverter circuits for notebook type personal computers with aims of making inverter circuits compact and highly efficient. Such an inverter circuit for a notebook type personal computer requires one leakage flux transformer and a resonance circuit on the secondary side per each cold-cathode fluorescent lamp, and has power of 5 W or so at a maximum.
Multiple cold-cathode fluorescent lamps are used in a surface light source such as a liquid crystal display backlight, and there is a demand of making the power of the associated inverter circuit greater accordingly.
There are multiple proposals on inverter circuits for high-power multi-lamp surface light sources. Many of the inverter circuits use multiple collector resonating circuits which are often used in the conventional inverter circuits. In one of the proposals, a single small leakage flux transformer is provided per two cold-cathode fluorescent lamps as shown in FIG. 2 for the purpose of reducing the overall cost for the inverter circuit.
When one wants a higher efficiency, however, it is effective to resonate the secondary side circuit as disclosed in U.S. Pat. No. 5,495,405. In this case, the collector resonating circuit and the resonance circuit present in the primary circuit interfere with each other, making it very difficult to adjust the circuit constant.
Since the exciting current which flows across the primary winding is used as the resonance current from the resonance circuit on the primary side according to the principle of the collector resonating circuit, the effect of improving the power factor cannot be utilized to a certain extent when the invention described in U.S. Pat. No. 5,495,405 invention is achieved by collector resonating circuits. In this respect, another exciting circuit or so which can extremely reduce the exciting current is frequently used.
In either case, those inverter circuits are each designed merely in such a way that multiple small high efficiency inverter circuits are laid out in proportion to the number of cold-cathode fluorescent lamps, and are thus complicated.
It is the step-up transformer and the drive circuit in the inverter circuit for a high-power surface light source that require the cost most, so that the required use of the step-up transformer and the drive circuit causes the overall cost for the inverter circuit to increase.
While it is necessary to achieve cost reduction for an inverter circuit for discharge lamps by reducing the number of step-up transformers and drive circuits by making the power of the step-up transformers greater, it is difficult to drive cold-cathode fluorescent lamps in parallel.
The difficulty arises from the following reason. A cold-cathode fluorescent lamp has a negative impedance characteristic such that the voltage falls as the current increases. Even with an attempt to drive cold-cathode fluorescent lamps in parallel, therefore, when one of the parallel-connected cold-cathode fluorescent lamps is lighted, this cold-cathode fluorescent lamp lighted first drops the lamp voltages of the other cold-cathode fluorescent lamps connected in parallel. As a consequence, all the cold-cathode fluorescent lamps except for the cold-cathode fluorescent lamp that is lighted first are not lighted.
As a solution to this problem, a scheme of stably driving multiple cold-cathode fluorescent lamps in parallel has been proposed by the present inventor in U.S. Ser. No. 10/773,230 (corresponding to Japanese Patent Application No. 2004-003740) as shown in FIG. 3, in addition to the suggested use of cold-cathode fluorescent lamps which can be lighted in parallel, such as an external electrode fluorescent lamp (EEFL).
As parallel driving of multiple cold-cathode fluorescent lamps becomes possible, a high-power step-up transformer becomes necessary to drive the transformers. In an inverter circuit for discharge lamps, like cold-cathode fluorescent lamps, which require a high voltage, it is very difficult to make the power of the step-up transformer higher for the following reason.
First, increasing the power of the step-up transformer requires that the transformer should be made larger. This naturally increases the thickness of the transformer, which is not allowed to become too thick due to the particular demand of designing liquid crystal display backlights thinner besides compactness.
Because the shape of the transformer greatly influences the parameters thereof and the relationship between the cross-sectional area of the magnetic path and the length of the magnetic path should be kept at a constant ratio, however, the shape of the transformer does not have a high degree of freedom. When a thinner design is sought out, the length of the magnetic path should be greater than the cross-sectional area of the magnetic path. This leads to a smaller coupling coefficient k of the transformer, resulting in a larger value of the leakage inductance Le (as defined by The Institute of Electrical Engineers of Japan (IEEJ)) to the self-inductance Lo. The term “leakage inductance” defined in books published by IEEJ differs from the same term “leakage inductance” obtained by the JIS measuring method. To distinguish the leakage inductances, therefore, the former leakage inductance is called “leakage inductance Le (IEEJ)”, and the latter is called. “leakage inductance Ls (JIS)”. Both leakage inductances can be mutually converted by an equation given below.
The leakage inductances have the following relationship.
The leakage inductance Le (IEEJ) is given byLe=(1−k)·Lo.
The mutual inductance M is given byM=k·Lo.
The leakage inductance Ls (JIS) is given by
      L    s    =            1                        1                      L            e                          +                  1          M                      +          L      e      
It is apparent that as the leakage inductance Le (IEEJ) increases, the leakage inductance Ls (JIS), which is an important parameter to constitute a resonance circuit on the secondary winding side, becomes larger.
In constructing a high efficiency inverter circuit described in U.S. Pat. No. 5,495,405, it is desirable that the leakage inductance Ls (JIS) should have the following relationship with the impedance Zr of the discharge lamp.|XL,≦|Zr|
This means that a high efficiency inverter circuit can be realized when the reactance of the leakage inductance Ls (JIS) at the operational frequency of the inverter circuit is nearly equal to or slightly smaller than the impedance of the discharge lamp. This relational equation applies effectively to an inverter circuit for a large surface light source as well as to an inverter circuit for a notebook type personal computer.
If multiple cold-cathode fluorescent lamps are driven in parallel with an increase in the power of the surface light source, therefore, impedance Zr of the discharge lamp is the impedance of the cold-cathode fluorescent lamps divided by the number of the cold-cathode fluorescent lamps and is thus a small value. The relationship between the leakage inductance Ls (JIS) and the impedance Zr indicates that a high efficiency inverter circuit can be realized when the reactance of the leakage inductance Ls (JIS) at the operational frequency of the inverter circuit is equal to or slightly smaller than the impedance of the discharge lamp. This means that the leakage inductance Ls (JIS) needed for transformers for a high-power inverter circuit should be small.
When the shape of the step-up transformer is restricted so as to match with the flat shape actually demanded for a liquid crystal display backlight, however, the leakage inductance Ls (JIS) should become large as explained above. It is very difficult to design a flat and high-power transformer.
Another important factor is the speed of a progressive wave which is generated on the secondary winding. First, as the shape of the transformer becomes larger with an increase in power, the self-resonance frequency of the secondary winding becomes lower. The self-resonance frequency of the secondary winding in the inverter circuit for cold-cathode fluorescent lamps is associated with the step-up effect and is therefore an important parameter. The relationship will be described in detail below.
The windings of a transformer are in a state of a distributed-constant as shown in FIG. 4 in a detailed illustration including the influence of the distributed capacitance. The influence of the distributed constant of the windings is analyzed in detail as a countermeasure against breakdown of a power transformer originated from the lightening surge as described in, for example, “Transformer in Power Device Course 5” (published by The Nikkan Kogyo Shimbun, Ltd.). It is known from the literature that the windings of a transformer form a delay circuit having a specific distributed constant. The influence of such a property appears noticeably when multiple very thin wires are wound up as done for the secondary winding of a step-up transformer for cold-cathode fluorescent lamps.
In the actual step-up transformer for cold-cathode fluorescent lamps, the distributed constant of the secondary winding appears around the self-resonance frequency or at a frequency higher than the self-resonance frequency. As the secondary winding forms a delay circuit, transmission delay of the energy occurs from that portion of the secondary winding which is close to the primary winding to that portion of the secondary winding which is far from the primary winding, as shown in FIGS. 5 to 7. This phenomenon is so-called phase-shift or phase modification wherein the phase is delayed gradually. The term “phase modification” is known in the field of motors or the like.
The phase modification in the present invention is called “phase-modifying transformer” by Electrotechnical Laboratory (currently, National Institute of Advanced Industrial Science and Technology) when authorized to do a subsidized research of Kanto Bureau of International Trade and Industry in Ministry of International Trade and Industry (currently, Kanto Bureau of Economy, Trade and Industry) in 1996. The phase modification phenomenon results in that the current phase of that portion of the secondary winding which is close to the primary winding becomes close to the current phase of the primary winding, so that a large portion of the flux generated on the primary winding penetrates the secondary winding, thus forming a close coupling portion, as shown in FIG. 8.
This structure noticeably appears in the vicinity of the frequency at which the leakage inductance Ls (JIS) of the secondary winding and the capacitive component on the secondary side resonate, but does not appear when no resonance takes place.
Therefore, the resonance of the leakage inductance Ls (JIS) of the secondary winding and the capacitive component on the secondary side is essential in the appearance of the structure of close coupling and loose coupling.
The current phase of the portion of the secondary winding which is far from the primary winding is delayed from the current phase of the primary winding, so that a large portion of flux leaks from the secondary winding, thus forming a loose coupling portion. At the loose coupling portion, as shown in FIG. 8, most of the flux that has penetrated from the primary winding leaks, so that the leakage flux leaks differently from that in the prior art and, even with the same leakage inductance, a larger amount of flux leaks at the loose coupling portion than that in the prior art. That is, a so-called extreme leakage flux is produced. (In FIGS. 5 to 8, not only 100% of the magnetic flux or more leaks, but also 35% of a magnetic flux of the opposite phase is generated.) Such flux leakage phenomenon differs from the behavior of the leakage flux in the prior art. FIG. 9 shows the behavior of the leakage flux in the conventional transformer illustrated for readers' reference.
As a signal which travels on the secondary winding with a distributed constant has a given propagation speed due to such a phase delay phenomenon, the signal has a given wavelength from the relationship with the drive frequency. The propagation speed is about several Km/sec for a transformer in an inverter circuit for cold-cathode fluorescent lamps. Consequently, a progressive wave is generated on the secondary winding of the transformer in the inverter circuit. Given that the wavelength of the progressive wave is λ, when the wavelength of ¼λ coincides with the physical length of the bobbin of the secondary winding, a resonance phenomenon similar to the resonance of an antenna or the resonance of an acoustic resonant body as shown in FIG. 10 occurs. In this case, the resonance frequency of ¼λ is the self-resonance frequency of the secondary winding itself, so that the resonance frequency of ¼λ can be known by actually measuring the self-resonance frequency of the secondary winding of the transformer.
In the general knowledge, the step-up ratio of the transformer becomes greater as the transformation ratio becomes larger. On the contrary, detailed observations show that such is not true at a frequency close to the self-resonance frequency. The transformer shows the maximum step-up operation at a frequency at which the self-resonance frequency, which is the resonance frequency of the self-inductance of the secondary winding and the distributed capacitance of the secondary winding (parasitic capacitance between windings), becomes equal to the operational frequency of the inverter. That frequency is the resonance frequency of ¼λ.
When the self-resonance frequency becomes lower than the operational frequency of the inverter, the transformer gradually loses the step-up operation. When the self-resonance frequency further drops and becomes a half the operational frequency of the inverter, the transformer does not make the step-up operation at all. This is because at the resonance frequency of ½λ, the current phase of the secondary winding at a far end portion which is apart from the primary winding is delayed by 180 degrees from, and becomes opposite to, the current phase of that portion of the secondary winding which is close to the primary winding.
When the self-resonance frequency becomes lower than the operational frequency of the inverter, various phenomena, such as suppression of the step-up operation and generation of a voltage of the opposite phase, may occur. In the general knowledge, however, the step-up operation has not been thought in such a concept.
That is, it is the conventional knowledge that the transformation ratio should simply be increased to gain the step-up ratio, so that an insufficient step-up ratio when pointed out is coped with winding the secondary winding more.
This measure however leads to excessive winding of the secondary winding, which often results in a lower self-resonance frequency of the secondary winding. Although the step-up ratio may be repressed due to the excessive winding of the secondary winding, it is often the case that when the proper step-up ratio is not obtained, an attempt is made to wind the secondary winding more to gain the step-up ratio. The excessive winding of the secondary winding, further lowers the self-resonance frequency. This results in a vicious circle of suppressing the step-up ratio more. As apparent from the above, the self-resonance frequency of the secondary winding of the transformer has a significance in the step-up transformer for cold-cathode fluorescent lamps and care should be taken not to make the self-resonance frequency too low.
From the viewpoint of the coupling coefficient, the self-resonance frequency can be set high to a certain degree by increasing the number of sections of the secondary winding of the transformer. Setting the number of sections larger means that the coupling coefficient becomes smaller and the leakage inductance becomes larger.
Because the impedance of a load to be driven in a high-power inverter circuit is low, the leakage inductance in a high-power transformer should be made smaller in proportion to the load. Therefore, there is a limit to increasing the number of sections. As the transformer becomes larger, the self-resonance frequency inevitably becomes lower, so that contradictory conditions should be satisfied to reduce the leakage inductance and acquire a transformer with a high self-resonance frequency. Needless to say, designing the transformer is difficult.
The secondary winding of the transformer has a distributed constant and forms a delay circuit. The secondary winding therefore has a characteristic impedance from the theory of a high-frequency transmission circuit. To form the ideal close coupling portion/loose coupling portion structure, the characteristic impedance which is determined by the size of the bobbin of the transformer, the cross-sectional area of the core, the magnetic path and the winding of the secondary winding should be matched with the impedance of the load of the discharge lamp.
Without impedance matching, an echo is generated, so that the ideal delayed waveform is not acquired, resulting in generation of a standing wave. As a result, the leakage flux on the secondary winding does not become uniform, disabling the achievement of the ideal conditions to ultimately minimize the core loss.
To reduce heat generated in a high-power transformer, the copper loss and the core loss should be minimized. However, with a requirement of a flat shape added to the difficult requirement that three conditions of the leakage inductance, the speed of the progressive wave (i.e., the self-resonance frequency) and the characteristic impedance should be met, it becomes harder to design a transformer which satisfies all the conditions at a time.
Several attempts have been made to achieve a high-power step-up transformer by connecting a plurality of transformers in parallel.
FIG. 18 shows an example of a discharge lamp which is driven with a pulse signal and is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-138097.
In the example, an attempt is made to realize a high-power step-up circuit by connecting both the primary windings and the secondary windings of a transformer which drives a discharge lamp to be driven with a pulse signal. In particular, a pulse transformer requires that the leakage inductance should be particularly small because a large leakage inductance disables the supply of a sharp pulse with a large value of di/dt.
Generally speaking, however, when transformers with very small flux leakage are connected in parallel, the current may flow between the secondary windings of the transformers and reduce the efficiency or heat may be generated due to variations in the characteristics of the individual transformers. In this respect, the example disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2000-138097 uses resistor components of the secondary windings of the transformers to disperse the load evenly over the individual transformers.
That is, the parallel connection of transformers essentially requires the reactance for parallel connection. With insufficient reactance, the load to be dispersed over the transformers does not become uniform, so that when multiple transformers are connected, the load is concentrated on some transformers.
When the reactance is given by a resistor component, reduction in efficiency by the generation of the Joule heat should be taken into consideration.
When a discharge lamp is driven with a sine wave of 40 KHz to 100 KHz as done for a cold-cathode fluorescent lamp, the leakage inductance larger than that needed for pulse driving is required to acquire the reactance for parallel connection. Conventionally, in the case of driving a cold-cathode fluorescent lamp, ballast capacitors are often connected in series as the ballast reactance. The step-up transformer in this case does not use the resonance of the secondary side circuit as used in U.S. Pat. No. 5,495,405. The transformers to be used in this case have a small leakage inductance and are of course unsuitable for parallel connection. In addition, the transformation ratio of transformers which are not resonated reflects on the step-up ratio directly, so that for parallel connection, the step-up ratio should be controlled strictly so as to have no variation.
FIG. 19 shows an example of parallel connection disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H10-92589, where the transformer has a small leakage inductance and the secondary side circuit is not resonated. In this case, when the secondary windings of the transformers are connected in parallel, the current that flows between the secondary windings may increase, generating heat.
To acquire parallel connection of transformers having small leakage inductance, therefore, a practical inverter circuit is difficult to design unless the parallel connection is made via ballast capacitors as shown in FIG. 20.